In this study we develop a tool to simultaneously invert multi-source magnetic transfer functions (TFs), including magnetotelluric (MT) tippers (with period ranging from a few minutes to 3 hours), solar quiet (Sq) global-to-local (G2L) transfer functions (TFs; with period ranging from 6 hours to 24 hours) of ionospheric origin, and magnetospheric global Q-responses (with period ranging from a few days to a few months). We further jointly invert the aforementioned multi-source TFs to constrain the local conductivity structures beneath three islands located in South Atlantic, Indian Ocean and North Pacific. The recovered conductivity profiles suggest upper mantle plumes beneath Tristan da Cunha and Oahu islands. Besides, our results indicate resistive lithosphere of different thicknesses beneath these three islands, showing a progressive thickening of oceanic lithosphere with age.